1. Field of the Invention
The invention is related to light emitting diodes (LEDs), and more particularly, to a high efficiency LED with an optimized photonic crystal extractor.
2. Description of the Related Art
A light emitting diode (LED) is a semiconductor device that emits light in a stimulated manner when electrically biased in the forward direction. This effect is a form of electroluminescence.
An LED is comprised of a chip of semiconducting material impregnated, or doped, with impurities to create a structure called a pn junction. When biased forwardly, electrons are injected into the junction from the n-region and holes are injected from the p-region. The electrons and holes release energy in the form of photons as they recombine. The wavelength of the light, and therefore its color, depends on the bandgap energy of the materials forming the pn junction.
As semiconductor materials have improved, the efficiency of semiconductor devices has also improved and new wavelength ranges have been used. Gallium nitride (GaN) based light emitters are probably the most promising for a variety of applications. GaN provides efficient illumination in the ultraviolet (UV) to amber spectrum, when alloyed with varying concentrates of indium (In), for example.
Unfortunately, most of the light emitted within a semiconductor LED material is lost due to total internal reflection at the semiconductor-air interface. Typical semiconductor materials have a high index of refraction, and thus, according to Snell's law, most of the light will remain trapped in the materials, thereby degrading efficiency. By choosing a suitable geometry for the LED, a higher extraction efficiency can be achieved.
FIG. 1 is a cross-sectional view of a homogeneous light-emitting material 10 that illustrates that a fraction of the light 12 emitted inside the material 14 is inside the escape cone 16 and can escape the material 10, while a large fraction of the emitted light 18 is trapped and reflected within the material 10. In this situation, the reflected light 18 is referred to as guided light modes, or guided modes, because the light 18 is confined within the device 10 and guided transversely within the material 10. One method for reducing the effects of the total internal reflection is to create light scattering or redistribution through random texturing of the surface of the device, which leads to multiple variable-angle incidence at the semiconductor-air interface of the device. This approach has been shown to improve emission efficiency by 9-30%, as a result of the very high internal efficiency and low internal losses, which allows many passes for light before it escapes from the device.
FIG. 2 is a cross-sectional view of a semiconductor LED 20 that illustrates this concept, wherein the top surface 22 of the LED 20 is textured, the bottom surface 24 of the LED 20 comprises a reflector, the air has a refractive index of n=1, and the semiconductor material of the LED 20 has a refractive index of n=3.5. The textured top surface 22 of the LED 20 is used to randomize light trajectories in a geometrical optics approach.
Another method to reduce the percentage of light trapped is to use a Resonant-Cavity LED (RCLED) or Micro-Cavity LED (MCLED). MCLEDs offer opportunities to create solid-state lighting systems with greater efficiencies than existing systems using “traditional” LEDs. As a result of incorporating a gain medium within a resonant cavity, MCLEDs emit a highly compact and directional light beam. The higher extraction efficiency and greater brightness of these devices are the main advantages of these technologies over conventional LEDs.
Extraction efficiency refers to the ability of the photons generated by a particular system to actually exit the system as “useful” radiation. This higher extraction efficiency is, however, limited to values in the 40% range as the micro-cavity structure also leads to very efficient emission into guided modes and leaky modes. Thus, it would be useful if these guided modes could be extracted.
As noted above, guided modes are modes that are guided in the device plane due to the index difference between the structure layers. Leaky modes are radiated through the layers, towards the air or substrate—for instance through a DBR. Leaky modes are usually lost as they undergo multiple total internal reflection at interfaces, travelling back and forth within the device, until their energy is dissipated by various loss mechanisms (e.g., metal mirror loss, free carrier absorption, re-absorption by the active layer which recycles energy by re-emission, although with some losses, etc.).
FIG. 3 is a cross-sectional view of a semiconductor LED 26 that illustrates radiating, guided and leaky modes, wherein the LED 26 includes a substrate 28, a buffer layer 30, and an active layer 32 including quantum wells (QWs) 34. Part of the emitted light is extracted 36 towards the air and forms radiative modes 36, part of the emitted light leaks 38 through the various layers of the device 26 into the substrate 28 and forms leaky modes 38, and part of the emitted light is reflected 40 in the active layer 32 (or in both the active layer 32 and buffer layer 30) and forms guided modes 40.
To obtain high efficiency LEDs, it is necessary to optimize direct mode emission 36 outside the structure, minimize the leaky mode emission, if such modes are present 38, and, if possible, re-emit the guided mode emission 40. The present invention aims at fulfilling this goal, in structures amenable to easy fabrication.
FIGS. 4A-4B and 5A-5B illustrate the structures (FIGS. 4A and 5A) and simulations (FIGS. 4B and 5B) of micro-cavity emission, via a bottom metal or distributed Bragg reflector (DBR) mirror, and a single interface with air at the top, in a gallium nitride (GaN) materials system. FIGS. 4B and 5B are angular emission diagrams inside the semiconductor, on a log scale, wherein FIG. 4B shows the emission of the structure of FIG. 4A, and FIG. 5B that of FIG. 5A. For both FIGS. 4B and 5B, the left half of the figure shows the transverse magnetic (TM) emission and its right half shows the transverse electric (TE) polarized emission. Only in-plane monochromatic dipoles are assumed.
In FIG. 4A, the structure includes a metal mirror 41, and an active layer 42 including quantum wells 43, wherein the structure is a 3λ/4 cavity with the quantum wells 43 placed at λ/4 (44) of the metal mirror 41. In FIG. 4B, arrow 45 shows emission towards the air, while arrow 46 shows emission towards the substrate. Also in FIG. 4B, braces 47 indicate extracted light, and brace 48 indicates guided modes.
In FIG. 5A, the structure includes a buffer layer 49, 7 period DBR mirror 50 and active layer 51 including quantum wells 52, wherein the structure is a λ cavity with the quantum wells 52 placed at λ/2 (53) of the 7 period DBR mirror 50. In FIG. 5B, arrow 54 shows emission towards the air, while arrow 55 shows emission towards the substrate. Also in FIG. 5B, braces 56 indicate extracted light, braces 57 indicate leaky modes, and brace 58 indicates guided modes.
Extraction efficiency from these structures is, respectively, in FIG. 4A, 31% and 24% in air, and in FIG. 5A, 44% and 27% in epoxy.
Difficulties are encountered in most materials systems when attempting to obtain large micro-cavity extraction improvements of LEDs. FIGS. 4B and 5B show the emission diagrams from which is extracted the expected efficiency of optimized GaN micro-cavity LEDs, and illustrate the following issues:                (i) The index contrast for materials epitaxially grown is quite limited, in particular for the very important nitride materials. This is why emission into many leaky modes is seen in the DBR mirror 50 structure of FIG. 5A, which make the DBR mirror 50 structure of FIG. 5A less efficient than the metal mirror 41 of FIG. 4A.        (ii) The displayed efficiencies are only possible because very thin structures were considered, leading to low-order cavities (as described in reference 10 below). It is difficult to obtain such thin active layers. For example, it is usually necessary to grow a thick (several microns) buffer layer of nitride on a substrate before growing good quality material for the active layer. While lifting off the nitride materials (buffer layer and active layer) from the substrate is already a delicate operation, there is, in addition, extreme difficulty in obtaining the thinner layers (i.e., further removing part or all of the buffer layer) bounded by good metal mirrors, which would lead to the excellent performance of the thin metal mirror structure shown in FIG. 4A.        
Thus, there is a need in the art for improved LED structures that provide increased light extraction efficiency. In addition, there is a need in the art for improved LED structures that optimize direct emission outside the structure, minimize leaky mode emission, and re-emit guided mode emission. Moreover, there is a need to provide such improved LEDs while retaining a planar structure, in order to render such structures amenable to easy fabrication. The present invention satisfies these needs, and aims especially at the case of LEDs that support numerous guided modes (e.g., LEDs whose thickness can not easily be made thin enough to make them support only one or a few modes). Finally, the present invention can enhance the directionality of light emission, which is a highly desirable property for some applications, such as LCD displays.